1. Field of the Invention
The present invention relates to an improved process for selective and precise etching and plating of conductive substrates, including a process for selective and precise etching and plating of substrates having irregular topography such as three dimensional substrates.
2. Background Art
Chemical etching is employed to fabricate a wide variety of materials. In general, such an etching process comprises application of a photoresist composition to a material of construction. A typical liquid-type photoresist composition contains a combination of a film forming resin or polymer and a photosensitive compound or a photoiniator dissolved or suspended in an organic solvent composition. After application to a substrate and evaporation of any solvent carrier, the photoresist is selectively exposed through use of a photomask and actinic radiation. By providing areas that are selectively opaque and others that are transparent to the radiation, a pattern is defined and transferred to the photoresist coating layer. The pattern is then developed by treatment of the coating layer with a developer solution. Those portions of the substrate that are exposed by development of the photoresist layer can be chemically milled, for example with a ferric chloride solution, to remove unwanted areas of the material of construction and thereby form a substrate with a structure the same as the pattern transferred through the photomask to the photoresist coating layer. After etching, the resist can be stripped from the substrate with a suitable stripper solution as is known in the art.
A photoresist can be either positive-acting or negative-acting. For a negative photoresist, those coating portions that are exposed to actinic radiation polymerize or cross link in a reaction between the photoinitator and polymerizable reagents of the resist composition. Consequently, those exposed portions are comparatively less soluble in a developer solution than unexposed portions. For a positive-acting photoresist, exposed coating portions are rendered soluble in the developer solution. For both types of resists, the solubility differences between exposed and unexposed coating portions permit construction of a patterned relief image and thus the transfer of an image upon subsequent processing steps.
Photoresist compositions are similarly employed to selectively metal plate or coat (e.g. paint) a surface. In general, a plating or coating step simply replaces the substractive etching step. Thus, a photoresist is applied to a substrate surface; the resist is exposed to provide selectively soluble portions of the photoresist coating; a developer is applied to bare selected portions of the substrate surface; those selected portions are plated or coated; and the remaining resist stripped from the substrate surface.
It is generally desired to apply a uniform photoresist coating on a substrate. This is particularly the case for milling or plating higher performance products where high precision processing steps are required. Uneven and insufficient resist coatings can operate as sites for penetration of an etching solution or plating solution. Uneven coatings also can result in irregular exposure of the resist, providing poor resolution of the developed image or incomplete development if the resist is underexposed.
Most prior processes for application of a photoresist composition on a substrate can be generally characterized as planar coating processes; that is, the resist is directly applied only along a single plane of the substrate rather than evenly across each surface of a multi-dimensional substrate. Such prior photoresist coating methods include spray coating, dip coating, roller coating, screen coating and dry film.
These planar application methods can be inadequate where the topography of the coated surface is irregular, for example where all surfaces of a three dimensional substrate are to be coated. Thus, a spray application will not apply resist in the same manner or amount to substrate surfaces that directly face the spray applicator as the resist that is applied to substrate surfaces that regress from or otherwise do not directly face the spray applicator. Common coating irregularities include insufficient coverage of all surfaces of the three dimensional substrate, regression of the photoresist from the substrate edges, thinning of the resist at substrates corners and drips and runs of the photoresist, all resulting in areas of uneven application, pinholes in the coating layer and/or uncoated areas.
These planar application methods also can be inadequate for applying a photoresist along a single plane where the substrate surface is irregular. For instance, photoresist will be applied in a different amount or manner in pitted areas than in more uniform areas of an irregular substrate surface. Such problems are significant where resolution demands are high. This is a recognized limitation to the use of dry film. Current dry film lamination application methods will bridge over surface irregularities, generally preventing satisfactory resolution of an etched image of a width of less than about 3 to 4 mils.
Thus, problems posed by uneven photoresist applications are of significant concern in the fabrication, plating or coating of higher performance products where chemical milling of narrow width substrates is required or where a plating or coating application must be strictly limited to a specific area.
Specifically, prior photoresist application methods have been inadequate or of limited application for lead frame manufacture. A lead frame is a sheet metal framework on which an integrated circuit is attached and electrically connected to an electronic printed circuit board. The lead frame provides a mounting surface for a microelectronic integrated circuit from which multiple conductive circuit "leads" laterally extend. Each lead is spaced from each adjacent lead to provide a separate conductive pathway. The top surface of the leads are selectively plated with a conductive metal such as gold or silver to electrically connect individual conductive pads of an integrated circuit to the board circuitry.
The lead frame primary structure is formed by metal stamping or chemically etching a suitable electrically conductive substrate such as a copper or nickel alloy foil. An etching process is preferred for higher performance applications as the process permits greater resolution and thus smaller lead dimensions.
Narrow lead widths are required in many lead frame applications. Each input or output of an integrated circuit requires a separate lead to provide a conductive path to board circuitry. The total number of inputs and outputs of an integrated circuit is known in the art as the "input-output count" or simply the "I/O count". To provide higher functionality, many integrated circuits have I/O counts of 200 or more. Manufacture of integrated circuits of even higher I/O counts is anticipated.
To accommodate such high function integrated circuits, lead frames having a sufficient number of leads or "lead count", i.e. 200 or more leads, are required. Further, it is generally desired not to increase lead frame surface area. Thus the need exists for lead frames having lead widths as narrow as possible to thereby provide the greatest number of leads in the smallest amount of space. However, conventional photoresist application methods such as dry film generally limit successful etching to lead widths not less than about 3 to 4 mils.
Prior photoresist application methods also can be inadequate in the selective plating process of the top surface of leads of a lead frame. For high signal speed applications, that is, lead frames used for electrically connecting electronic devices that operate at frequencies of 10 megahertz ("MHz") or greater, it is crucial that the conductive metal is selectively plated only on the top surface of the lead, and that metal is not deposited along the lead's edges, sidewalls or any other surface of the lead frame not intended for plating. Metal deposition along such surfaces will cause problems in subsequent packaging reliability and compromise electronic performance. Conventional planar photoresist applications methods can insufficiently coat lead frames edges or sidewalls and thus fail to restrict or limit plating to lead top surfaces.
Electrodeposition involves a process of electrophoresis which is the motion of charged particles through a liquid medium under the influence of an applied electrical current. The deposition is conducted in an electrolytic cell where the conductive surface to be coated serves as one electrode. A charged polymer suspended typically as a colloidal emulsified dispersion in a liquid medium is electrophoretically deposited on an oppositely charged electrode. Deposition of a positively charged polymer on a negatively charged cathode is referred to as cataphoresis while deposition of a negatively charged polymer on a positively charged anode is known as anaphoresis.
Electrodeposition of photosensitive coatings is generally known. See, for example, U.S. Pat. Nos. 3,738,835; 3,954,587; 4,029,561; 4,035,273 and 4,035,274. Electrodeposition of photosensitive polymer compositions is described in U.S. Pat. Nos. 4,592,816 and 4,414,311, both incorporated herein by reference.
Most known photoresist compositions are not suitable for use in an electrophoretic lead frame manufacturing process. For example, typical etching and plating solutions, for example a ferric chloride or cupric chloride etching solution and silver or gold cyanide plating solutions, are quite aggressive and can leach a photoresist coating, resulting in etching or plating on surfaces not intended for treatment. Such irregular etching and/or plating is unacceptable for manufacture of many substrates, for example lead frames used in high performance applications such as connecting an integrated circuit with a high I/O count or a circuit operated at high frequencies.